Pressure die casting a metal alloy makes it possible to obtain a finished part directly at casting and is used in very large series to manufacture many parts that are part of mass consumption products such as supports or casings, in particular smart phones, computer tablets, cameras, but also parts subjected to high stress, in particular in the automobile industry, such as fuel injection rails, or hydraulic distributors without being limited to these examples. Typically, the parts coming from this method have a complex form, combining zones of highly variable thickness and comprising zones of low thickness. These parts must be produced in compliance with strict constraints in terms of aspect and precision, while still maintaining production speeds that are compatible with mass production. According to this method, the material forming the future part is brought to a suitable temperature, then is injected under pressure into the cavity of a mould that is resistant to the casting temperature and that comprises two metal dies, or more. The mould is preheated to a temperature less than the temperature of the injected material, in such a way that said material cools in contact with the walls of the mould. The part is cooled in the mould to a release temperature, temperature at which the mould is opened and the part, solidified, is ejected from the mould. Before producing another part, the mould being opened, the surfaces forming the cavity of said mould are sprayed with a release product, generally an aqueous product, which ensures the absence of catching or sticking of the future cast part on the walls of the mould. The mould is then closed and the cycle restarts. By way of an example of implementation, the metal is injected at a temperature between 550° C. and 650° C. according to the grade of the material and the type of casting: liquid phase or thixocasting, while the mould is preheated to a temperature of 300° C. FIG. 1, relative to the prior art shows, in FIG. 1A, an example of a thermal cycle corresponding to the method described hereinabove, showing the change in the temperature (102) on the surface of the cavity of a mould as a function of time (101), change obtained by installing a temperature probe on one of the surfaces delimiting the cavity of the mould, or by means of an infrared thermography of said surface, said mould being formed from tool steel of the DIN 1.2343 type (AISI H11, EN X38CrMoV5-1) and being intended for the casting of a thin magnesium alloy part, with the projected surface of the imprint being 200×300 mm2. According to this prior art, the mould is preheated by means of a circulation of oil in conduits made for this purpose in the mould. During the step (110) of casting, the metal is injected into the mould. Said mould is preheated to a nominal preheating temperature (105), frequently about ⅓ to ½ of the casting temperature expressed in ° C., in such a way that said metal solidifies in contact with the walls of the mould. During a step of (120) releasing, the mould is opened, then the part is extracted from the mould during a step of (130) ejection. During these steps, the temperature of the cavity is maintained close to the preheating temperature. During a step (140) of spraying, a release agent is sprayed onto the surfaces of the moulding cavity. The mould is then closed and the means for regulating the temperature of the latter are implemented during a step of heating (150) in order to bring the latter to the nominal preheating temperature (105), this step of heating continues until the restarting of the cycle. The step of spraying (140) substantially reduces the temperature of the surfaces of the moulding cavity, in such a way that the conventional means of heating the mould, in particular via the circulation of oil, do not make it possible to reach the suitable nominal preheating temperature (105), while still complying with the production speeds sought.
Indeed, in the case of heating via the circulation of oil, the thermal energy transmitted by the oil to the mould is according to the difference in temperature between the mould and the oil, in such a way that the closer the temperature of the mould comes to the temperature of the oil the less effective this transfer is. The oil circulating at a temperature slightly greater than or equal to the nominal preheating temperature, the time for reaching this new temperature is conditioned by the heat exchanges between the oil and the mould, which take places over durations that are not compatible with the speeds sought.
Thus, in FIG. 1B, the temperature reached on the surfaces of the moulding cavity after the step of preheating, decreases from cycle to cycle. By way of example, for a temperature of the oil in circulation of 250° C., and a sought nominal preheating temperature of 230° C., the effective preheating temperature (106) during the 10th cycle is only 195° C. and 185° C. during the 14th cycle (107). By way of example, the duration of the cycle is about one minute, the duration of the ejection step (130) is about 8 seconds and the duration of the step (140) of spraying and of closing the mould is about 10 seconds. As these durations vary according to the cast material, the volume and the complexity of the part as well as the means implemented. The speeds that correspond to theses times do not allow for the rising of the temperature of the mould via heat exchange with the oil in circulation. Indeed, the rise in the preheating temperature sought, in the allotted timeframe, implies a thermal transfer power of several tens of KW, which cannot be achieved via an exchange with oil in circulation, more particularly when the difference in temperature between the heating oil and the mould is reduced. It is also not possible to achieve the dissipation of such heating power over the moulding surfaces via a conductive exchange with heating resistances.
Thus, according to these same measurements, the maximum heating speed of the moulding surfaces during the step (150) decreases as the difference in the temperature between the oil and the mould decreases, to descend to speeds of about a few degrees per minute over the last tens of degrees of preheating.
As the temperature of the moulding surfaces of the cavity is cooler, the metal cools faster in contact with the latter and loses fluidity more quickly which results in quality defects of the part produced, in particular aspect defects or missing material, more particularly in the zones of low thickness.
Document US 2016/101460 discloses a casting method comprising a step of spraying by a release agent of the moulding surfaces of a cavity delimited by the two portions of a mould. During the step of spraying, in order to prevent thermal shocks on the moulding surface and the risks of cracking, due to the high cooling speed imposed by the spraying of the release agent, this document recommends a pre-cooling of said surfaces using the circulation of a fluid in the mould.
Document US2016/101551 describes a mould with autonomous heating and cooling, the heating being carried out via induction by means of field windings extending into hoses made in the mould. This document does not describe operations of spraying moulding surfaces, or operations of controlling the cooling of these surfaces during the spraying thereof.